I have an idea/suggestion. How can I share it with you?

How can I translate Viewnior?

You can do that on Launchpad. I import all translations once before each release.
If you are interested in translating every new release, you can follow the Announcements feed.

Where is “Set As Wallpaper”?

Wallpaper support for GNOME 3, GNOME 2, XFCE, LXDE, FluxBox, Nitrogen, and Puppylinux is available since Viewnior 1.2.
In older versions it is an optional feature (disabled by default), which can be enabled by running ./configure –enable-wallpaper. Note that (before Viewnior 1.2) this works only under GNOME environment.